Method of purifying low-carbon ferrochrome

ABSTRACT

METHOD FOR LOWERING THE NITROGEN, CARBON AND OXYGEN CONTENT OF FERROCHROMIUM ALLOY BY HEATING THE ALLOY UNDER VACUUM.

3,725,051 METHOD OF PURIFYING LOW-CARBON FERROCHROME Cecil GeorgeChadwick, Lewiston, N.Y., assignor to Union Carbide Corporation, NewYork, N.Y. No Drawing. Filed Oct. 14, 1970, Ser. No. 80,811 Int. Cl.C22c 1/06, 39/14 US. Cl. 75-1305 1 Claim ABSTRACT OF THE DISCLOSUREMethod for lowering the nitrogen, carbon and oxygen content offerrochromium alloy by heating the alloy under vacuum.

This invention relates to the removal of nitrogen, carbon and oxygenfrom low carbon ferrochromium alloys. More particularly this inventionis directed to the purification of ferrochromiurn alloys by vacuumfurnacing.

Low carbon ferrochromium alloy, intended for use as an alloy additionagent to steel is produced by re acting molten chrome ore and lime withferrochrome silicon alloy in an electric furnace to produce moltenferrochromium alloy which is thereafter cast into slabs or ingots toprovide ferrochromium alloy in massive form. The massive alloy issubsequently crushed to suitable industrial size such as 2 inches andfiner. The ferrochromium alloy in this form is dense, homogeneous andeasily handled. Such alloy ordinarily contains in addition to iron andchromium, from about 0.10 to 0.13% nitrogen, 0.022 to 0.05% carbon and0.05 to 0.12% oxygen. Such nitrogen, carbon and oxygen contents aregenerally undesirably high for additions to be used in the making ofhigh performance alloy steels.

It is therefore an object of the present invention to provideferrochromium alloy having very low nitrogen, carbon and oxygencontents.

Other objects will be apparent from the following description andclaims.

A method in accordance with the present invention comprises heatingferrochromium metal under vacuum at elevated temperatures to provideferrochromium containing not more than about 0.02% nitrogen, 0.01%carbon and 0.07% oxygen.

In the practice of the present invention, ferrochromium alloy obtainedby the solidification of molten ferrochromium for example containing 71to 74% Cr, 25 to 28% Fe, 0.10 to 0.13% N 0.022 to 0.05% C and 0.05 to0.12% is heated under vacuum at temperatures in the range of 1300 to1400 C. at pressures of from 60 to 500 microns. Suitable furnacing timesare from to 40 hours, preferably from 30 to 40 hours.

As a result of the foregoing treatment, the nitrogen content of theferrochromium can be reduced to less than 0.02%, the carbon can bereduced to less than 0.01% and the oxygen can be reduced to less thanUnited States Patent 0 0.07%. This result, i.e. the concurrent removalof nitrogen, carbon and oxygen, is surprising and leads to theproduction of a highly desirable material. For example, ferrochromiumalloy treated in accordance with the present invention can be readilyutilized in vacuum meliing operations to produce high performance alloystee s.

The following examples will further illustrate the present invention.

EXAMPLE I Chromium ore and lime were charged to an electric furnace. Theforegoing constituents were then reacted with ferrochrome silicon toproduce molten ferrochromium alloy. The molten ferrochromium alloy wasteemed and cast into slabs. The massive ferrochromium alloy wassubsequently crushed to obtain lumps of alloy about 2 inches in size.Approximately 15.0 lbs. of the lump ferrochromium containing about 71%Cr, 26% Fe was placed in a vacuum furnace and heated at about 1385 C.for various times up to forty hours at an absolute pressure of about 100microns. Analysis of the alloy before and after the vacuum furnacingtreatments is shown in Table I.

From the foregoing data it can be seen that after a vacuum furnacingtreatment of forty hours the nitrogen content of the alloy was reducedto about one tenth of its initial value, the oxygen content was reducedto about one-half of its initial value, and the carbon content wasreduced to one-fifth of its initial value.

What is claimed is:

1. A process for lowering the nitrogen, carbon and oxygen content ofparticulated low carbon ferrochromium alloy which comprises heating theparticulated ferrochromium at a temperature in the range of about 1300to 1400 C. under vacuum of about 500 to 100 microns for a period of timefrom about 20 to 40 hours whereby a product containing not more than0.02% nitrogen, 0.01% carbon and 0.07% oxygen is obtained.

References Cited UNITED STATES PATENTS 72,541,153 2/1951 Chadwick14s-13.1x 2,624,671 1/1953 Binder 126 3,523,021 8/1970 Khitrik 75-1305 xL. DEWAYNE RUTLEDGE, Primary Examiner I. E. LEGRU, Assistant Examiner

